The disease chytridiomycosis looms over the world’s amphibians like a grim reaper. Along with habitat loss, pollution, and other problems, this fungal infection is a central reason one third of Earth’s amphibians are threatened with extinction. Chytridiomycosis began killing amphibians in the 1980s, baffling scientists who had yet to name it. It has since hammered species in Australia, the Americas, Europe, Africa, and Asia.

When it struck blue poison dart frogs and other species at the Smithsonian’s National Zoo in the 1990s, former Zoo pathologists Don Nichols and Allan Pessier worked with fungus specialist Joyce Longcore of the University of Maine to isolate and describe the mystery pathogen.

Nichols had studied it since 1991, after seeing diseased tissue samples from Arroyo toads while working for the National Institutes of Health. He consulted experts in algae and single-celled animals called protozoa, but the pathogen was neither. The Zoo outbreak allowed him and Pessier to examine fresh samples of the infection under the microscope. It looked like fungus. Longcore confirmed their suspicions and helped describe the new species (in a new genus), which they named Batrachochytrium dendrobatidis in 1999.

Vietnamese mossy frog. (Jessie Cohen /NZP)

Nicknamed Bd, it is usually just called “chytrid fungus.” But Earth teems with friendlier chytrid fungi. None but Bd harms amphibians—or anything else with a backbone. These fungi comprise the phylum Chytridiomycota, and most of them decompose dead things, while fewer parasitize plants, other fungi, or invertebrates. Bd, the black sheep among its kin, can kill amphibians in days or weeks by destroying their delicate skin.

Its microscopic zoospores—round clear blobs with long wiggling tails—spread through water. They infect the surface layer of amphibian skin, each developing into a growing single-celled body called a thallus. The thallus makes a sac of new zoospores and releases them through tiny tubes to begin the cycle again.

The unlucky amphibian hosts often grow sluggish and begin shedding sheets of dead skin. Their epidermis may thicken or thin in various spots, redden on the belly, or develop ulcers. The infection is just skin deep, but skin plays a vital role for frogs and their relatives—it regulates their water, oxygen, and electrolytes. Scientists still puzzle over exactly how Bd inflicts damage. They hope to learn whether the fungus secretes a cell-destroying toxin.

Another puzzle is the rapid spread of Bd. Research suggests it came from Africa, possibly with frogs exported for labs and pet stores, or even on the shoes and gear of biologists and ecotourists. Scientists also suspect that global warming encourages its growth and virulence. But a recent study suggests that neither idea—Bd spreading quickly from a few spots, or getting a boost from climate change—fully explains its leaps and bounds.

Scientists do know that some amphibian populations or species resist the fungus, while others fall swiftly and completely. Bd figured notoriously in wiping out Panamanian golden frogs. Presumed extinct in nature, these banana-yellow frogs are now bred for conservation and
displayed at the Zoo’s Reptile Discovery Center. But other species—or individuals within species—can carry the fungus with few or no symptoms.

These survivors might have microscopic guardian angels. Studies reveal that certain bacteria found naturally on amphibian skin kill or ward off the fungus, and that amphibian populations carrying lots of these bacteria may stand stronger against chytridiomycosis. Scientists hope such findings will lead to treatments for wild populations; currently, they use antifungal medicines on captive amphibians, but Bd lurks in streams and lakes, waiting for any healthy amphibians they reintroduce.

New treatments may arise, since a team at the University of Idaho cracked Bd’s genetic code in 2008. The next step is to discover which genes make the fungus deadly.

For now, many amphibians depend on captive breeding programs of organizations like the National Zoo to evade extinction. The world’s frogs, salamanders, and their kin can’t leap or wriggle past this immense hurdle without our help.